Seafloor observations in natural electromagnetic fluctuations diffusing into the earth have been made in the vicinity of the Pacific Rise at 12¿N and 21¿N to gather information on the electrical conductivity structure of the oceanic basement near a spreading ridge. At 21¿N the close proximity of land (Baja California) introduces a noticeable distortion of the EM fields, and consequently the magnetotelluric sounding for this area must be interpreted with caution. At both locations the conductance of the upper 200 km appears exceptionally high and the conductivity rises rapidly below, reaching 1 S m-1 or more at 350 km. The statistical significance of a prominent conductivity increase near a depth of 22.5 km detected in the inverted data from 12¿N and of a less developed one at 30 km in the case of 21¿N is discussed. Records of the magnetic variations of 21¿N taken above the spreading center, compared to records from 40 and 120 km to the west, reveal a conspicuous enhancement of the high-frequency horizontal variations in a direction slightly west of magnetic north. We interpret this distinctive feature as the signature of an electric current sheet concentrated in an extremely high conductance, relatively shallow layer electrically bridging the two seafloor areas adjacent to the spreading center. An approximate calculation suggests a conductance in excess of 3000 S per meter of ridge crest in the close vicinity of the accretion center, equivalent to 1 to 8 km of molten basalt, depending on temperature (1000¿-1200¿C?) and depth (2-10 km?). This structure is consistent with seismic refraction information for the same area and with stratigraphic reconstruction of a fossil accretion center in Samail ophiolite. An interpretation in terms of a well-developed magma chamber is irresistable, although confirmation of the existence of this feature as well as improvement of its resolution by means of additional EM data would be of major interest.